Implement default help-echo for mode line.

Implement default help-echo for mode line.

* lisp/bindings.el: Remove explicit help-echo from format-mode-line.
(mode-line-front-space, mode-line-end-spaces)
(mode-line-misc-info): New variables.
(mode-line-modes, mode-line-position): Move the default value to
the variable definition.
(mode-line-default-help-echo): New defcustom.

* src/buffer.c (buffer_local_value_1): New function, split from
Fbuffer_local_value; can return Qunbound.
(Fbuffer_local_value): Use it.
(Vmode_line_format): Docstring tweaks.

* src/xdisp.c (calc_pixel_width_or_height): Use Fbuffer_local_value.
(note_mode_line_or_margin_highlight): If there is no help echo,
use mode-line-default-help-echo.  Handle the case where the mouse
position is past the end of the mode line string.

* doc/lispref/modes.texi (Mode Line Data): Use "mode line construct"
terminology for consistency.